Arius (; Koine Greek: Ἄρειος, romanized: Áreios; 250 or 256 – 336) was a Cyrenaic presbyter and ascetic. He has been regarded as the founder of Arianism, which holds that Jesus Christ was not coeternal with God the Father, but was rather created by God the Father. Arian theology and its doctrine regarding the nature of the Godhead showed a belief in radical subordinationism, a view notably disputed by 4th century figures such as Athanasius of Alexandria.

Constantine the Great's formal decriminalization of Christianity into the Roman Empire entailed the convention of ecumenical councils to remove theological divisions between opposing sects within the Church. Arius's theology was a prominent topic at the First Council of Nicaea, where Arianism was condemned in favor of Homoousian conceptions of God and Jesus. Opposition to Arianism remains embodied in the Nicene Creed, described as "a deliberately anti-Arian document." Nevertheless, despite concerted opposition, Arian churches persisted for centuries throughout Europe (especially in various Germanic kingdoms), the Middle East, and North Africa. They were suppressed by military conquest or by voluntary royal conversion between the fifth and seventh centuries.

Arius's role as the sole originator of Arian theology has been disputed by historians such as Rowan Williams, who stated that "Arius' role in 'Arianism' was not that of the founder of a sect. It was not his individual teaching that dominated the mid-century eastern Church." Richard Hanson writes that Arius' specific espousal of subordinationist theology brought "into unavoidable prominence a doctrinal crisis which had gradually been gathering[...] He was the spark that started the explosion. But in himself he was of no great significance." The association between Arius and the theology titled after him has been argued to be a creation "based on the polemic of Nicene writers" such as Athanasius of Alexandria, a Homoousian.